The thawing of timber, either prior to barking or in connection therewith, is indispensable in wintertime in northern regions, particularly when barking birch timber. Presently, such thawing is most commonly effected by warm water, which is supplied into the barking drum. This procedure, or so-called wet barking, results in large quantities of waste water and, in particular, in high contents of biologically oxygen-consuming substances in the waste water. In order to avoid this, the majority of timber treatment installations are provided with a separate thawing conveyor which feeds the timber into the barking drum. On the barking conveyor, the timber is thawed either with warm water or with steam. The first alternative results in waste water quantities at least equal to those incurred in wet barking, but the content in the waste water of substances causing biological oxygen consumption is clearly less. Steam thawing is even more advantageous since it requires a smaller waste water quantity. However, the problems encountered in steam thawing are the achievement of adequate thawing capacity and the high investment required for the barking conveyor.
Efforts to reduce the equipment costs required in timber treatment, and to make the use of energy more efficient, have given rise to the concept of steam thawing in the barking drum. The concept is not entirely novel: such drums have been constructed heretofore, into which steam is supplied in addition to water. In them, the purpose of the steam has mostly been to furnish additional heat during the coldest time of the year. Thawing which is effected entirely by steam has not hitherto been achieved, in the first place owing to the circumstance that it has not been possible to distribute steam over a long enough region within the drum. If it were possible to supply steam into the barking drum in the desired region, it would be possible to improve substantially the thawing capacity, and thereby the barking capacity.